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SubjectRe: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition
On (01/18/16 14:39), Junil Lee wrote:
> record_obj() in migrate_zspage() does not preserve handle's
> HANDLE_PIN_BIT, set by find_aloced_obj()->trypin_tag(), and implicitly
> (accidentally) un-pins the handle, while migrate_zspage() still performs
> an explicit unpin_tag() on the that handle.
> This additional explicit unpin_tag() introduces a race condition with
> zs_free(), which can pin that handle by this time, so the handle becomes
> un-pinned.
>
> Schematically, it goes like this:
>
> CPU0 CPU1
> migrate_zspage
> find_alloced_obj
> trypin_tag
> set HANDLE_PIN_BIT zs_free()
> pin_tag()
> obj_malloc() -- new object, no tag
> record_obj() -- remove HANDLE_PIN_BIT set HANDLE_PIN_BIT
> unpin_tag() -- remove zs_free's HANDLE_PIN_BIT
>
> The race condition may result in a NULL pointer dereference:
> Unable to handle kernel NULL pointer dereference at virtual address 00000000
> CPU: 0 PID: 19001 Comm: CookieMonsterCl Tainted:
> PC is at get_zspage_mapping+0x0/0x24
> LR is at obj_free.isra.22+0x64/0x128
> Call trace:
> [<ffffffc0001a3aa8>] get_zspage_mapping+0x0/0x24
> [<ffffffc0001a4918>] zs_free+0x88/0x114
> [<ffffffc00053ae54>] zram_free_page+0x64/0xcc
> [<ffffffc00053af4c>] zram_slot_free_notify+0x90/0x108
> [<ffffffc000196638>] swap_entry_free+0x278/0x294
> [<ffffffc000199008>] free_swap_and_cache+0x38/0x11c
> [<ffffffc0001837ac>] unmap_single_vma+0x480/0x5c8
> [<ffffffc000184350>] unmap_vmas+0x44/0x60
> [<ffffffc00018a53c>] exit_mmap+0x50/0x110
> [<ffffffc00009e408>] mmput+0x58/0xe0
> [<ffffffc0000a2854>] do_exit+0x320/0x8dc
> [<ffffffc0000a3cb4>] do_group_exit+0x44/0xa8
> [<ffffffc0000ae1bc>] get_signal+0x538/0x580
> [<ffffffc000087e44>] do_signal+0x98/0x4b8
> [<ffffffc00008843c>] do_notify_resume+0x14/0x5c
>
> Fix the race by removing explicit unpin_tag() from migrate_zspage().
>
> Signed-off-by: Junil Lee <junil0814.lee@lge.com>

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: <stable@vger.kernel.org> [4.1+]


migrate_zspage() was introduced in 4.1
https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/mm/zsmalloc.c?id=refs/tags/v4.1

thank you.

-ss

> ---
> mm/zsmalloc.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index e7414ce..0acfa20 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1635,8 +1635,8 @@ static int migrate_zspage(struct zs_pool *pool, struct size_class *class,
> free_obj = obj_malloc(d_page, class, handle);
> zs_object_copy(free_obj, used_obj, class);
> index++;
> + /* This also effectively unpins the handle */
> record_obj(handle, free_obj);
> - unpin_tag(handle);
> obj_free(pool, class, used_obj);
> }
>
> --
> 2.6.2
>

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